On-site surveys of weed populations provide information on the relative occurrence and density of weeds that can be useful to growers in that region. Data generated by weed surveys can aid in the management of weed issues by monitoring the movement of problem weeds and forecasting areas susceptible to infestations. Currently, on-site surveys are often performed on a small scale, within single fields or counties. Questionnaire surveys are helpful for assessing relative abundance but do not always provide detailed information on weed distribution in time or space. A survey was conducted annually in Ohio from 2013 through 2017 in 49 counties with soybean [Glycine max (L.) Merr.] production to assess the late-season occurrence of horseweed [Conyza canadensis (L.) Cronquist]. The objectives of this research were to: (1) determine the frequency, level of infestation, and distribution of C. canadensis in soybean fields in the primary soybean-producing Ohio counties over 5 yr; and (2) identify significant spatial clusters or movement trends over time. Conyza canadensis was encountered in each county from 2013 through 2017. Spatial cores of interest, or counties identified as having significant levels of C. canadensis infestations or a lack thereof relative to surrounding counties, were identified in all years except 2017. The lowest frequency of C. canadensis encountered at all rating levels occurred in 2017, which coincided with second-highest frequency of infestations (highest density level) among years. There was no distinct distribution or pattern of C. canadensis movement within the state from year to year, but there was an increase in counties with infestations over time compared with the early years of the survey when many counties had few to no infestations. These results suggest that C. canadensis persists as a common and troublesome threat to Ohio soybean producers and that growers should continue making C. canadensis management a priority when developing weed control programs.

Current recommendations for the control of glyphosate-resistant horseweed [Conyza canadensis (L.) Cronquist var. canadensis] in soybeans [Glycine max (L.) Merr.] consist of comprehensive herbicide programs, which often include herbicide applications outside the soybean growing season. Integration of cover crops with herbicides could potentially improve C. canadensis control and allow for a reduction in herbicide inputs. Two separate field studies were conducted from 2016 through 2018 with the objectives of: (1) determining the effect of planting date and seeding rate of a cereal rye (Secale cereale L.) cover crop on C. canadensis population density and control in the subsequent soybean crop; and (2) determining whether the cover crop could replace a fall herbicide treatment or allow for a reduction in the use of spring-applied residual herbicides. There was no effect of rye planting date, late September versus late October, on C. canadensis density in either study. In 2016 to 2017, C. canadensis density was greater in the absence of a rye cover crop in both studies, but otherwise not affected by seeding rates of 50 versus 100 kg ha−1. In the 2017 to 2018 season, the presence of rye resulted in an increased C. canadensis density in the spring residual herbicide study (Study I), and had no effect in the fall herbicide study (Study II). Conyza canadensis densities were lowest in the treatments where a comprehensive spring residual or fall herbicide treatment had been applied, averaged over rye planting date and seeding rate. Earlier-planted rye at a higher seeding rate produced the most biomass but did not result in lower C. canadensis densities. These results suggest that cereal rye planted at a density of 50 kg ha−1 as a cover crop before no-till soybeans may be sufficient to reduce glyphosate-resistant C. canadensis plant density, but cannot be relied upon to reduce the need for fall herbicide treatments and spring residual programs.

Greenhouse experiments that used capillary mat subirrigation to maintain constant soil moisture and to supply fertilizer continuously were conducted to evaluate the effects of sorghum or rye residue on early growth of barnyardgrass and velvetleaf. The separate effects of root residue and of shoot residue were compared to the combined effects of root plus shoot residues and to an uncovered soil control. Residues included as nontoxic controls were leached shoot tissue and poplar excelsior. Shoot residue, leached shoot tissue, and poplar excelsior were surface-applied on an equal light transmittance basis such that mass of poplar excelsior > shoot residue > leached shoot tissue. The presence of rye root residue delayed emergence of barnyardgrass. Surface-applied residues tended to decrease barnyardgrass height, but velvetleaf stem length was greater in treatments with surface residue. Although cover crop shoot residues had little effect on weed growth after 18 d, weed growth decreased in the presence of cover crop root residues and poplar excelsior.

Common cocklebur root and shoot interferences were investigated as factors causing soybean yield reductions in the field. Significant decreases in soybean yield resulted from common cocklebur shoot interference when the two were grown together with the common cocklebur roots confined in plastic bags. Soybean yield decreased further when common cocklebur roots were not confined, but this decrease resulted, in part, from increased common cocklebur shoot growth. Wire mesh cylinders of varying diameters, placed in gaps in the soybean rows and designed to spatially restrict soybean shoot growth, reduced yield more than did gaps alone. When similar cylinders lined with black plastic were placed next to the soybean rows to restrict both within-canopy light and space for soybean shoot growth, soybean yield was reduced most (44%) with concomitant changes in stem height, stem diameter, lodging, and branches, nodes, and pods/plant. Interference of common cocklebur with soybeans resulted primarily from shoot interference, and competition for light within the soybean canopy implicated as the major factor causing the soybean yield reduction.

The study of weed life cycles, reproductive strategies, and the soil seed bank is emphasized in the undergraduate weed science course at Ohio State University as central to an understanding of the survival of weeds in the environment. A laboratory exercise was conducted every spring and fall academic quarter from 1991 to 1993 to demonstrate the effects of long-term cropping and soil disturbance histories on weed seed banks and aboveground weed communities. Five sites with diverse histories of culture were sampled; these included a field cultivated in vegetables under continuous conventional tillage for 59 yr, a field cultivated in field corn under continuous no-tillage for 11 yr, a 24 yr-old turfgrass research farm, a 70 yr-old forest, and a section of the forest border. Students conducted a survey of the weeds growing at the sites and separated and identified seeds from soil samples over a 3-wk period in weekly 2-h laboratory periods. Students wrote reports interpreting the data based on their knowledge of the site histories, weed life cycles, and weed seed production and longevity characteristics. The data were consistent over academic quarters as well as with published research, indicating that the survey and soil sampling techniques provided a reasonably accurate representation of the weed flora and soil seed populations. Weeds found growing at the sites were primarily summer annuals at the vegetable site, and a mix of summer and winter annuals, biennials, and perennials at the remaining sites. Annual weeds dominated the seed banks of all sites with common lambsquarters, pigweed spp., and common purslane being the most commonly found seeds. The presence of most seeds in the soil could be explained by a combination of species seed production and seed longevity characteristics and species abundance in the standing community. Interpretation of the data required students to integrate and apply lecture material and provided an excellent thinking exercise.

Photosynthesis and growth responses to irradiance level during growth were compared in soybean (Glycine max L. Merr. ‘Century’) and three broadleaf weeds to determine if these responses were associated with differences in shade tolerance among species. In response to reduced irradiance during growth, leaf thickness of all species decreased, while chlorophyll content per unit leaf volume and photosynthetic rate per unit leaf volume, measured at low irradiance, increased. Soybean and common cocklebur (Xanthium strumarium L. #3 XANST) also exhibited a decrease in soluble proteins on a leaf volume basis under reduced irradiance, and common cocklebur further exhibited a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPcase) protein per unit leaf volume. Decreased irradiance during growth did not alter the content of RuBPcase or other soluble proteins per unit leaf volume in jimsonweed (Datura stramonium L. # DATST) or velvetleaf (Abutilon theophrasti Medic. # ABUTH). The superior shade tolerance of common cocklebur compared to the other species was attributed in part to the levels of RuBPcase and other photosynthetic proteins in leaves developed at low irradiance.

Common cocklebur, jimsonweed, and velvetleaf were grown with soybeans in the field to determine how soybean interference affects weed canopy architecture. Common cocklebur had more leaves within the soybean canopy than jimsonweed or velvetleaf. At the end of the season, common cocklebur leaf area was distributed evenly below and above the top of the soybean canopy, while nearly all the jimsonweed and velvetleaf leaf area was above the soybean canopy. Common cocklebur exhibited more shade tolerance than jimsonweed or velvetleaf by also maintaining leaves in the shade within the soybean canopy. Differences among these weeds in leaf distribution within the soybean canopy were not related to differences in abscission of the lower leaves but to a differential response of lower axillary buds to soybean shading. Growth from lower axillary buds in jimsonweed and velvetleaf was strongly inhibited by soybean interference, but interference had little effect on lower axillary bud growth in common cocklebur. Axillary bud growth in the lower canopies of both common cocklebur and soybeans, and their similarity in height, caused these plants to compete for the same aboveground niche. However, common cocklebur had more extensive axillary growth along the lower stem than soybeans, which may allow it to compete for resources in this niche more aggressively than soybeans. Velvetleaf and jimsonweed did not share the same aboveground niche with soybeans due to the placement of their leaves above rather than within the soybean canopy. The lower branching characteristics and apparent shade tolerance of common cocklebur may be important factors in the superior competitive ability with soybeans compared to jimsonweed and velvetleaf.

Field experiments were established at Columbus and near South Charleston, OH to determine the effects of giant ragweed population density on soybean yield and to characterize the development of giant ragweed grown in 76-cm soybean rows. An economic threshold was calculated for Ohio using a common treatment for giant ragweed control in soybean. A cost of $41/ha was estimated for a farmer to apply 0.56 kg/ha bentazon plus 0.28 kg/ha fomesafen plus COC (1.25% v/v). Assuming a soybean value of $0.22/kg, the cost of control was equivalent to 5.4 and 7.1% of the soybean yield in 1991 and 1992, respectively, which corresponded to the yield loss caused by 0.08 and 0.03 giant ragweed plants/m2. The competitiveness of giant ragweed can be at least partly attributed to its ability to initiate and maintain axillary leaves and branches within the shaded confines of the soybean canopy.

Greenhouse experiments were conducted to determine the statistical precision of estimating herbicide dose-response treatment effects by covariance analysis (ANOCOVA) relative to standard analysis of variance (ANOVA). Analyses of corn seedling response to the translocated herbicides fluazifop-P, sethoxydim, and quizalofop at 10 to 60 g ai ha-1 indicated that treatment effects were estimated with 26 to 116% greater precision by ANOCOVA than ANOVA. Covariance analyses of treatment effects for corn response to the contact herbicides paraquat, acifluorfen, and lactofen at 50 to 300 g ai ha-1 gave 8 to 13% greater precision than ANOVA. Gains in precision by ANOCOVA for all experiments were generally greatest when shoot dry weight was analyzed as the response variable and pretreatment fifth leaf length served as the covariate.

Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.

Field studies were conducted in 1990 and 1991 to determine the effects of corn planting date and hairy vetch control method on the efficacy of fall-planted hairy vetch as a weedsuppressive cover crop for no-till corn. Glyphosate controlled hairy vetch when applied at the early bud growth stage (April), but hairy vetch residue provided no weed control compared to the weedy check. Mowing was not an effective means of suppressing hairy vetch at the early bud stage. Untreated hairy vetch reduced weed biomass 96% in 1990 and 58% in 1991 but reduced yield over 76% in April-planted corn. There was no competition of untreated hairy vetch with corn when corn planting was delayed until May or June (mid- or late-bloom growth stages of hairy vetch). Corn planted in May into untreated hairy vetch yielded similarly to corn planted in a no-cover weed-free check.

Lower leaves of greenhouse-grown common cocklebur and velvetleaf were shaded to 5% of full light over a 12-d period while upper leaves remained exposed to full light to determine weed foliar and branching responses to partial shading similar to that encountered in soybean crops. Shading increased lower leaf senescence and specific leaf area, and decreased branch length and number of second-order leaves in both species compared to unshaded controls. Common cocklebur branched more extensively along the lower portion of its stem than velvetleaf under both shaded and unshaded conditions. Upper leaves of partially shaded velvetleaf were held in a more perpendicular position to the light source beginning 3 days after treatment (DAT) compared to upper leaves of unshaded plants. Shading of lower leaves caused an increase in upper (unshaded) leaf area beginning 3 and 6 DAT in velvetleaf and common cocklebur, respectively. Petiole length of upper leaves also increased in response to shading in both species. Total plant dry weight at 12 DAT was unaffected by shading in velvetleaf but was reduced 10% by shading in common cocklebur. While common cocklebur maintained greater lower shoot growth in the presence of shade than velvetleaf, there was a greater change in upper leaf angle by velvetleaf in response to shading than by common cocklebur. These results support previous field observations of apparent greater shade tolerance of common cocklebur compared to velvetleaf and indicate that both species have the ability to compensate for shading of lower leaves by altering upper shoot growth.

Field studies were conducted to determine the effects of cultivated oats planting pattern on early canopy shape and growth of cultivated oats and wild oats, in part to test the assumption of radial plant canopy expansion on which previous theoretical models of crop-weed interference models have been based. Cultivated oats density was kept constant as the pattern rectangularity was varied, and single wild oats plants were centered within each pattern. Individual plant canopies, photographed from above 31 days after emergence (DAE), were radial for wild oats in all crop planting patterns and for cultivated oats planted in triangular and square planting patterns. Canopy radius perpendicular to the crop row axis in rectangular patterns was similar to canopy radius along the same cardinal axis in equidistant patterns, but was reduced along the crop row axis, resulting in a rectangular canopy shape and decreased canopy area in rectangular compared to equidistant patterns. Cultivated oats dry weight and leaf area at crop flowering (64 DAE) also decreased with increasing rectangularity of crop planting pattern. Reductions in cultivated oats growth in rectangular patterns were associated with earlier intraspecific interference and delayed crop canopy closure in rectangular compared to equidistant patterns. Wild oats leaf area and tiller number 64 DAE decreased with more equidistant crop planting patterns, consistent with reduced canopy area 31 DAE and earlier crop canopy closure in equidistant patterns. The data suggest that individual oats canopy expansion during early growth is essentially radial and also support previous theoretical predictions of crop planting pattern effects on weed suppression.

Giant ragweed germination is delayed by both a physiological dormancy of the embryo (embryo dormancy) and an inhibitory influence of embryo-covering structures (covering structure-enforced [CSE] dormancy). To clarify the roles of embryo and CSE dormancy in giant ragweed seedling emergence timing, we conducted two experiments to address the following objectives: (1) determine changes in germinability for giant ragweed dispersal units (hereafter “involucres”) and their components under natural burial conditions, and (2) compare embryo and CSE dormancy alleviation and emergence periodicity between successional and agricultural populations. In Experiment 1, involucres were buried in crop fields at Columbus, OH, periodically excavated, and brought to the laboratory for dissection. Involucres, achenes, and embryos were then subjected to germination assays at 20 C. In Experiment 2, temporal patterns of seedling emergence were determined at a common burial site. Reductions in embryo and CSE dormancy were compared with controlled-environment stratification followed by germination assays at 12 and 20 C, temperatures representative of soil conditions in spring and summer. Results indicated that overwinter dormancy loss involved sequential reductions in embryo and CSE dormancy. CSE dormancy, which may limit potential for fatal germination during fall, was caused by the pericarp and/or embryo-covering structures within the pericarp. In Experiment 2, successional populations emerged synchronously in early spring, whereas agricultural populations emerged throughout the growing season. Levels of embryo dormancy were greater in the agricultural populations than the successional populations, but CSE dormancy levels were similar among populations. In 12 C germination assays, embryo dormancy levels were positively correlated with time required to reach 95% cumulative emergence (run 1: r = 0.81, P = 0.03; run 2: r = 0.76, P = 0.05). These results suggest that late-season emergence in giant ragweed involves high levels of embryo dormancy that prevent germination at low temperatures in spring.

A greenhouse study was conducted to determine the effects of sublethal dicamba concentrations in the nutrient media on hydroponically grown tomato plants. Tomato leaf area was the most sensitive vegetative growth parameter measured in response to dicamba concentrations, ranging from 0 to 22 µg/L. Leaf area was reduced 31 and 76%, and specific leaf weights, a relative measure of leaf thickness (g/cm2), increased 26 and 121% after 30-d exposure to dicamba concentrations of 2.2 and 22 µg/L, respectively. In long-term experiments conducted until plants produced first ripe fruit, regression analysis indicated leaf area reductions of 8 and 66% from initial dicamba concentrations of 1 and 10 µg/L, respectively. Reductions in total fruit fresh weight were highly correlated (r = 0.93) with leaf area reductions caused by dicamba. A hyperbolic regression model gave predicted losses in fruit fresh weight per plant of 6% at 1 µg/L dicamba and 73% at 10 µg/L dicamba (r2 = 0.87). Results generally indicated that the level of dicamba in the nutrient media of hydroponically grown tomatoes that produced no observable effect was ≤ 1 µg/L.

Field trials were conducted with spring-sown rye and field pea cover crops to determine the effect of five rye–pea proportions and three seeding rates (high, medium, and low) on weed suppression during cover crop growth. Measurements on weed and cover crop growth were taken approximately 2 mo after seeding when cover crops were killed. Cover crops were killed by mowing in 1996 and by undercutting in 1997 and 1998. Cover crop biomass, averaged over rye–pea proportion, was highest in 1998 at 4.3 million tons (MT)/ha (high seeding rate) and lowest in 1997 at 1.5 MT/ha (low seeding rate). Cover crops of pure rye or rye–pea mixes suppressed weeds more effectively than did pure pea. Dominant weeds were ladysthumb, smooth pigweed, smallflower galinsoga, and common lambsquarters. Ground cover by weeds ranged from a low of 2% (rye–pea mixes) to a maximum of 73% (pure pea). Cover crop mixes of 50% or more rye seeded at the high rate gave the best weed suppression.

Giant ragweed exhibits a high degree of polymorphism among individual plants in seed size, shape, spininess, and color. These features may play an important role in giant ragweed seed survival and predation avoidance; however, they are difficult to evaluate because of lack of quantification methods. A computer imaging technique was developed for describing and classifying giant ragweed seeds using digital images of the seed top and side views. Seed samples collected from 20 different giant ragweed plants (classes) were mounted and digitally scanned. Quantitative features were extracted from the seed images, including color, width, height, area, and seed perimeter. A polygon (convex hull) of the seed image based on the seed outline was constructed, from which spininess indices were developed. Fisher's linear discriminant with normalized nearest neighbor classification was used to classify randomly selected images of individual seeds according to class (maternal origin), using the extracted features as a database. The best classification rate achieved was 99%, with 138 out of 140 seeds correctly matched using data from both the top and side views. Seed features were easily extracted and varied from 1.2- to 4.5-fold among classes. Area and perimeter measurements varied least within classes but varied most among classes, suggesting that these features discriminate effectively among seeds from different plants in giant ragweed. Convex hull area : seed area ratio, using the seed top view images, was the best index of seed spininess, aligning well with visual assessment and providing greatest discrimination among classes. This experiment shows that in the case of giant ragweed, seeds from different plants are distinguishable in an objective and quantitative manner. This imaging technique can be applied to identification of seeds from different species and to studies on variable seed morphology within a species.

Interest in using crop competitiveness as an integrated weed management tool is increasing. Our objective was to describe traits that could be sources of the competitiveness we previously observed in grain sorghum grown in association with shattercane, which is a common annual weed and a close relative of the crop. Such information could aid in developing management practices for cultivated sorghum to improve its competitiveness with weeds. A bioassay was conducted to compare emergence of the crop and the weed in the greenhouse, and vegetative growth was monitored for 31 d in a within-row competition study. Results described a crop that competed well with the weed and other crop plants and agreed with studies showing that relative time of emergence influenced competitiveness. The mechanism by which grain sorghum emerged before the weed was a by-product of domestication that reduced glumes surrounding the wild-type seeds. This could be shown experimentally by hulling shattercane seeds, which then emerged almost as quickly as the grain sorghum. When planted in the grain sorghum row, shattercane plants from hulled seeds decreased the number of leaves and the root mass of the crop. Similarly, the time between emergence of the crop and emergence of shattercane was lessened by planting shattercane seeds early, and this increased the leaf number of the weed and shoot mass of the crop. It might be possible to increase weed suppression in grain sorghum by using management practices, such as more equidistant crop planting patterns that exploit the competitiveness already present, but which is being lost to interactions among crop plants.

A field study was conducted to determine the effects of giant ragweed emergence time and population density on corn grain yield, giant ragweed seed production, and giant ragweed predispersal seed losses. When weeds and crop emerged concurrently, hyperbolic regression of percent corn yield loss on giant ragweed population densities of 1.7, 6.9, and 13.8 weeds per 10 m2 gave a predicted loss rate of 13.6% for the first weed per 10 m2 in the linear response range at low densities and a maximum yield loss of 90% at high weed densities. Crop yield loss response to weed density was linear when giant ragweed emerged 4 wk after corn, and the regression coefficient indicated a yield loss rate of 1% per unit increase in weed density. A larger proportion of the variation in corn yield loss was explained by weed density (r2 = 0.99) than by weed biomass (r2 = 0.81). There was a positive linear relationship between giant ragweed seed production and weed density at each weed emergence time. When giant ragweed emerged with corn, regression equations for 1997 and 1998 gave a predicted seed rain of 146 and 238 seeds m−2 per unit increase in weed density, respectively. In both years when giant ragweed emerged 4 wk after corn, predicted seed rain was 16 seeds m−2 per unit increase in weed density. Viability of total giant ragweed seed was 56 and 38% in 1997 and 1998, respectively, and was not affected by weed emergence time or weed density. Feeding by insect larvae accounted for 13 to 19% of giant ragweed seed viability losses. Granivorous insects infesting giant ragweed seed were identified as a fruit fly (Diptera: Tephritidae), two weevils (Coleoptera: Curculionidae), and a moth (Lepidoptera: Gelechiidae).

Late-season giant ragweed emergence in Ohio crop fields complicates decisions concerning the optimum time to implement control measures. Our objectives were to develop a hydrothermal time emergence model for a late-emerging biotype and validate the model in a variety of locations and burial environments. To develop the model, giant ragweed seedlings were counted and removed weekly each growing season from 2000 to 2003 in a fallow field located in west central Ohio. Weather data, soil characteristics and geographic location were used to predict soil thermal and moisture conditions with the Soil Temperature and Moisture Model (STM2). Hydrothermal time (θHT) initiated March 1 and base values were extrapolated from the literature (Tb = 2 C, ψb = −10 MPa). Cumulative percent emergence initially increased rapidly and reached 60% of maximum by late April (approximately 400 θHT), leveled off for a period in May, and increased again at a lower rate before concluding in late July (approximately 2,300 θHT). The period in May when few seedlings emerged was not subject to soil temperatures or water potentials less than the θHT base values. The biphasic pattern of emergence was modeled with two successive Weibull models that were validated in 2005 in a tilled and a no-tillage environment and in 2006 at a separate location in a no-tillage environment. Root-mean-square values for comparing actual and model predicted cumulative emergence values ranged from 8.0 to 9.5%, indicating a high degree of accuracy. This experiment demonstrated an approach to emergence modeling that can be used to forecast emergence on a local basis according to weed biotype and easily obtainable soil and weather data.